The present invention relates generally to pneumatic tires and more specifically to radial carcass tires having a textile overlay structure located radially outwardly of their belt assembly.
Radial carcass tires having a reinforcing member disposed radially outwardly of the belt assembly comprising textile cords oriented at small angles with respect to the mid-circumferential plane of the tire have been found to be particularly durable when subjected to high revolution speeds.
Such a reinforcing member, referred to as an overlay ply, can be interposed between the radially outermost ply of the belt assembly and the tread and comprise one or more wraps having a width which is about equal to that of the widest of the belt plies.
Alternatively, the overlay ply can consist of two separate axially spaced apart ply portions either disposed radially outwardly of the belt assembly such as to cover the edges of the radially outermost belt ply or interposed between the belt plies such as to extend between the edges thereof.
Another possibility of assembling an overlay ply consists in interposing a helically wound cord or single yarn, which has been coated with elastomeric material, between the radially outermost ply of the belt assembly and the tread. As winding a single cord is time consuming, it has been proposed to assemble the overlay ply from a 5 to 30 mm wide, helically wound strip, made from cord reinforced elastomeric material, located radially outwardly to the belt plies.
The usual reinforcing material for the overlay plies is nylon, which has a Youngs modulus of about 6,000 MPa. Such a low modulus is necessary to accommodate the expansion of the belt during the shaping and vulcanizing steps.
European patent application 412,928 discloses a textile overlay structure reinforced with cords being made of materials showing on a homogeneous specimen a modulus of at least 15,000 MPa. The cords have a twist multiplier between 6 and 14 and at least one of the yarns included in the cords has a twist of at least 14 turns per inch, in abbreviation TPI (550 turns per meter, in abbreviation TPM). The use of high modulus cords in the overlay ply leads to tires including less material for a given reinforcing strength, hence the flatspot contribution from the overlay ply is reduced. Additionally, lighter, cooler running tires are obtained, which have increased high speed performance.
U.S. Pat. No. 4,893,665 discloses a cord comprising (a) at least two yarns each of which consists only of aramid filaments; and (b) a single core yarn which is not twisted together with or around any other yarn and consists of at least one filament selected from the group consisting of polyamides having a structure which is at least partially oriented and polyesters having a structure which is at least partially oriented such that a force of about 4 grams per denier is required to elongate said core yarn to 107% of its beginning length, said aramid yarns being adjacent to one another and twisted about said core yarn but not about one another, the denier of said core yarn being in the range of 5% to 30% of the sum of the deniers of said aramid yarns, said cable having a twist multiplier in the range of 5 to 12, and the ratio of twist in the aramid yarns to the twist in the cable being in the range of 1.0 to 2.0.
Referring to FIGS. 3 and 4 there is illustrated an example of a cord 40 manufactured in accordance with the ""665 patent. In the example shown the cord comprises three yarns 41,42,43 each of which consists only of aramid filaments. It is understood though that a cord according to the invention only needs to have at least two yarns consisting only of aramid filaments. As used herein xe2x80x9cconsists ofxe2x80x9d or xe2x80x9cconsists only ofxe2x80x9d means that no filaments of any material other than that specified are present in a yarn. A single core yarn 44 is disposed such that each of the aramid yarns 41,42,43 is adjacent to at least one other aramid yarn and is twisted about the core yarn, but the aramid yarns are not twisted about one another. That is to say, the aramid yarns completely surround the core yarn. The core yarn consists of at least one filament selected from the group consisting of polyamides such as nylon. As used herein a xe2x80x9ccore yarnxe2x80x9d is understood to be a yarn that is not twisted together with, or twisted around any other yarn, although the filaments of a core yarn may be twisted together with one another. In a preferred embodiment the core yarn consists of at least one filament of at least partially oriented nylon, and in a most preferred embodiment the core yarn consists of at least two, (usually more than one hundred), filaments of a nylon which has a structure that is at least partially oriented, said nylon filaments being twisted together with one another.
U.S. Pat. No. 4,155,394 discloses a textile cord comprised of a plurality of dissimilar plies of individual yarns helically cabled together, having one to six plies of aramid yarn, having a modulus, at one percent elongation and at a twist of about one turn per inch, in the range of 250 to 600 grams per denier and one to twelve plies individually selected from polyester and/or nylon yarns having a modulus, at one percent elongation and at a twist of about one turn per inch, in the range of 20 to 100 grams per denier. The yarns are cabled by twisting together helically about one another to form a cord having a twist of cabled yarns, or plies, in the range of two to 16 turns per inch, where the aramid yarn has a residual twist in its ply in the range of one to six turns per inch opposite the cord twist and said polyester or nylon yarn has a residual twist in its ply in the range of 0 to 0.5 turns per inch opposite the cord twist, so that (A) at an initial cord elongation, under an initial longitudinal tensile stress, the primary load bearing ply is the polyester or nylon, as such yarn stretches while said aramid yarn extends without appreciable stretch, and (B) after additional elongation of the cord, under appreciable additional longitudinal tensile stress, the primary load bearing yarn is the aramid as it reaches substantially full extension.
Referring to FIG. 5A, there is shown an unstretched cord 51 of helically cabled dissimilar yarns having a twist of about eight turns per inch comprised of one yarn of aramid 52 having a residual twist of about six turns per inch and two plies or yarns of nylon 53 having a residual twist of 0.5 turns per inch. In FIG. 5B, as the cord 51 is initially stretched, the load is primarily taken by the polyester or nylon cord 53 as they elongate by stretching in an axial fashion whereas the essentially inextensible aramid yarn 52, instead of stretching, simply elongates by beginning to straighten out. In FIG. 5C, as the 51 is stretched further in the longitudinal direction, the polyester or nylon yarn 53 continues to elongate by stretching, until the aramid yarn 52 essentially reaches the limit of is elongation by straightening and then becomes the principal load-bearing member of the cord.
It is an object of the present invention to provide a pneumatic tire with an overlay structure, which has excellent high speed properties, good comfort and rolling resistance, reduced flatspotting and produces low noise.
The invention provides a tire as defined in the claims. More specifically the invention provides a pneumatic tire comprising a radial ply carcass, a tread disposed radially outwardly of the crown region of the carcass and a crown reinforcing structure interposed between the tread portion and the crown region of the carcass in circumferential surrounding relation to the carcass, whereby the crown reinforcing structure includes a belt assembly having at least a first, radially innermost and a second, radially outermost belt ply, each of the belt plies comprising reinforcement cords extending parallel to one another in each belt ply, and the cords in the first belt ply making with the cords in the second belt ply opposed angles with respect to the equatorial plane of the tire, as well as a textile overlay structure consisting of a cord reinforced helically wound elastomeric strip extending transversely over the belt assembly and making an angle of between 0 degrees and 5 degrees with the equatorial plane of the tire. The cords include yarns of aramid and nylon twisted together and have, on the graph representing their elongation versus the load applied, a tangent of at most 5 cN/tex. % at elongations up to 4% and an inclination above 10 cN/tex. % at elongations above 6% after shaping and curing of the tire. The tire is preferably a passenger tire, runflat or not.